Commutation of direct-current dynamo-electric machines.



N0. 784,000. PATENTED FEB. 28, 1905.

G. S. DUNN.

COMMUTATION OF DIRECT CURRENT DYNAMO ELECTRIC MACHINES. APPLICATION IILED MAY 7, 1902. RENEWED SEPT. 12, 1904.

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No. 784,000. PATBNTED FEB. 28, 1905.

0. s. DUNN. v GOMMUTATION 0F 0111001" CURRENT DYNAMO ELECTRIC MACHINES.

APPLIOATION FILED MAY 7. 1902. RENEWED SEPT.12, 1904.

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7 Vfwmdws: Iii Vania?) I maz f z/gi izii gym 1 UNITED STATES Patented February 28, 1905.

PATENT OEETCE.

GANO S. DUNN, OE AMPERE, NElV JERSEY, ASSIGNOR TO OROOKER- WHEELER COMPANY, OF AMPERE, NE\V JERSEY, A CORPORATION OF NEW JERSEY.

COMMUTATION OF DIRECT-CURRENT DYNAIVlO-ELECTRIC MACHINES.

SPECIFICATION formingpart of Letters Patent No. 784,000, dated February 28, 1905.

Application filed May '7, 1902. Renewed September 12, 1904. Serial No. 224,180.

1'0 (LU whom, it nan/y concern.-

Be it known that I, GANo' S. DUNN, a citizen of the United States of America, and a resident of Ampere, Essex county, New J ersey, have invented certain new and useful Improvements in the Oommutationof Direct-Ourrent Dynamo-Electric Machines, of which the following is a specification.

The armature-coils of direct-current dynamo-electric machines may be divided into three classes-namely, first, those approaching a brush; second, those receding from a brush, and, third, those in contact with a brush. The current in the approaching and the receding coils is equal, but opposite in direction. The accomplishment of the reversal of the coils in their turn on passing the brush is called commutation. The success of the commutation is determined by its complete accomplishment before the leading terminal of a coil leaves the brush. \V hen commutation is not accomplished properly, the coil that is to take its place with the receding coils has not been brought into a corresponding electrical condition at the instant when it is inserted into the path of the current through the receding coils, and the sudden enforcement of this correspondence at the moment the commutatorbar leaves the brush causes a spark or burning of the brush and bar which generates heat and in time destroys the smooth surface of the commutator, causes abnormal wear, reduces the efiiciency of the machine, and makes its operation uncertain.

In the discussion of the various methods of commutation which follows, it will be assumed for simplicity that the current flows from the brush into the coil under consideration and that the commutator is moving under the brush "from right to left.

There are several means of effecting commutation. Among these are shifting the brushes toward one of the magnetic poles of the machine, so that the commutation will take place in a region Where the coil is influenced by the magnetic field. By generating in a coil while its terminals are short-circuited by a brush a local electromotive force this field causes,the rapid dying out of the current originally flowing and the building up of current in the opposite direction. By the time the leading terminal is ready to pass outt'rom under the brush the electrical condition of the coil is so nearly like that of the receding coils that at the moment of its being inserted into the path of the current through the receding coils there is no disturbance. Other means of effecting connnutation are special forms of winding, whereby a neutralizing and reversing electromotive force is generated in the leads connecting the coil with the commutator-bars to accomplish the reversal of the commutating-coil, this electromotiveforcebeing produced by magnets separate from the field-magnets or by carrying a load under one of the poles of the machine in itspath be tween the coil and a commutator-bar.

The above-described means and various known modifications thereof, all belong to what I term magnetic commutation. The. reversal is accomplished by an clectromotive force generated in the commutatingcoil or some part of its local circuit when short-circuited by the brush. There is a second generic method of commutation which 1 call resistance commutation, since it accomplishes the reversal ol the current in the commutah ing-coil by oli'ering alternative paths by which the current may reach the terminals 01 the commutating-coil, the relation of resistances in these paths being rapidly changed as the terminals of the commutatingcoil pass through the region of the brush-contactin such manner as to tend to cut oil the current entering the leading terminal, and to facilitate passage of current from the brush into the trailing terminal or the coil. \Vith substances of high resistivity and contact resistance, as where carbon or graphite brushes are employed, there is a strong throttling efl'ect exerted upon the current passing into the leading terminal of the commutating-coil, as the overlap between the brush and the commutator-bar of the lead ing terminal is diminished, which tends to cut off current from going into the bar of the leading terminal, and thereby to establish the tall amount of the normal current in the coil from the bar connected with the trailing terminal before the bar connected with the leading ter- 1 ininal of the coil passes out of contact with the brush. At the moment therefore when the leading terminal of the coil is disconnected from the brush and the coil is inserted into the path of the current through the receding coils there is no troublesome shock.

Most direct-current dynamo-electric machines at present in use commutate by combined magnetic and resistance methods. For reasons that it is not necessary to enumerate the limit of commutation by these methods or combinations thereof has practically been in certain proportions of the machinery and not only renders its cost out of proportion to its performance, but beyond a certain point renders the construction impracticable. No suitable substance has been found the resistivity and contact resistance of which is higher than that of carbon and its modifications, and the prospects of development along this line are not promising.

My invention without diminishing the value of magnetic commutation increases indelinitely the effectiveness of resistance commutation, so that much higher voltages than are now commonly employed with direct-current d ynamo-electrie machinery can be successfully employed. changes to be made in the structure of machines attended by reduction of cost.

My method of commutation consists in increasing step by step the number of brusheonmnitator contacts between the line and the advancing terminal of a coil which is being COlillDHtfltCtl as the coil approaches the position of a receding coil, thus finally forming a path of current with maximum resistance which leaves and returns a multiplicity of times into the commutator-surface.

l have shown the invention applied to simple multiple circuit or loop windings; but it is equally applicable to what are known as two-circuit or wave windings and to the various modilieations of each of these classes of windings.

In the accompanyingdrawings, which form a part of my s 'iecilication, Figure 1 is a side It also enables very important elevation showing portions of a commutator and a lnush-holder with my improved brush applied. Fig. 2 is a plan view of the same parts. Fig. 3 is a plan view of a modification in which the insulation between the dead brushes is slanting with respect to the commutator-bars. Fig. a is a side view of a further modification in which the brushes overlap one another. Fig. 5 is a plan view of the same, and Fig. (3 is a diagram showing approximately the operation of the invention.

Referring to Figs. 1, 2, and 3, the commutator A is supposed to revolve in the direction of the arrow. The brush-holder B, of any suitable construction, supports a brush in its socket U. The ordinary brush I), of carbon or graphite or other suitable material, is supported in the socket, as usual, so as to form electrical contact with the connnutator-bars 1;, connected with the conimutating coils 1/. (Shown only in Fig. (3.) Located in the socket 1 (J on the trailing side of the live brush 1) is a plurality of brushes IQ, of carbon or graphite or other suitable material, which are insulated from each other and from the live brush by suitable insulation F-suchas sheets of mica, paper, or equivalent insulation-that may be wrapped around the brushes E, so as to inclose them. The brushes l lareinsulated not only from the live brush and its holder, but from every other part of the machine, so that they are completely dead except for their contact with the commutator-bars. Hence they are called dead brushes.

The progress of a coil in commutation by my method that is to say, the electrical connections and condition of a coil as it occupies successively the positions of coils u, 1, 2, 3, 4, 5, (3, T, and 1' will now be (,liscussed.

Referring to the diagram Fig. (3, coil 1/ is receiving current from its left-hand or lHltling terminal which liows out at the righthand or trailing terminal and into the coils to the right. The direction and strength of this current are the same as that which is in the neighboring approaching coils. \Vhen the coil (6 reaches the position of the coil 1, it is short-circuited through the live brush; but, owing to its inductance, current continues to flow in the same direction as before, although of a diminished value. \Yhen' the coiln reaches position i, it is short-circuited through a dead brush. its advancing terminal is connected to the live brush 1) through the dead brush 1*), and its trailing terminal is eonnected to the live brush l)directly and also through coil 1. In each of these three connections there is one brush-toconnnutator contact from the live brush. Since the contact between the live brush and the commutator is common to all of the circuits shown in the drawings. it will not be further referred to. Also, while the neutral point is shown as at position 3, it maybe located farther to the right or to the left. Considering now coil 1/ when in position 2., current tends to How from the live brush in at the trailing terminal; but this, which would el'l'ect a reversal of the coil,

lDO

is opposed by the self-induction thereof. Current also tends to flow from the live brush and in at the leading terminal; but this is opposed by the two additional surface contacts between the commutator-bar and the first dead brush and this brush and the next commutator-bar. As the coil (6 moves to the left the number of these surface contacts between the live brush and the leading terminal increases step by step, there being two added contacts for each dead brush-that is, the facility of receiving current at the leading terminal is progressively diminished because of the increasing number of contact surfaces through which such current has to pass. On the other hand, the facility of receiving current at the trailing terminal from the coils to the right is not diminished. This tends to reverse the coil gradually or in steps, corresponding to the number of dead brushes. This I term fractional commutation. In order to obtain full benefit of the dead brushes, their pitch should be approximately that of the commutator-bars-that is to say, the distance between the centers of the dead brushes should be practically that between the centers of the commutatorbars. If, for example, the pitch of the dead brushes should be half that of the co111mutator-bars, there would be positions where two dead brushes cover one bar, and only one pair of contact surfaces would be effective instead of two. If the pitch of the dead brushes was intermediate between onehalf of the pitch and the full pitch of the commutator-bars, then a portion of the brushes, less than half, would be rendered useless in certain relative positions of the commutatorbars and the brushes. The-case is similar if the brush-pitch exceeds the bar-pitch.

To state concisely mymethod of commutation, it may be said that when the coil is shortcircuited through the brush there is a moment at which the contact resistance is balanced at the two terminals of the coil and that the resistance is thereafter progressively increased on one side and not increased on the otherthat is to say, after the moment at which the resistance is balanced at the two terminals of the coil the resistance at the leading terminal is progressively augmented by the addition of contact resistance, while the inductive reactancc of the coils is dying away. In this way a path of current with maximum resistance is formed which departs from and returns a multiplicity of times into the commutator-surface. By following the pitch described 1 am enabled to extend this effect indefinitely. An effect to be noted clue to this method of commutation is that not only into the advancing terminal of one, but into the advancing terminals of a number of successive coils, a gradtially-increasing resistance is introduced in such a way that at each terminal a resist-ance of different amount to that at any other terminal is produced at a given time, forming as many paths of current as there are different resistances.

It will be understood that each dead brush may be compound-that is,compose of a num ber of non-insulated parts.

In Fig. 3 the planes of separation between the brushes are inclined relatively to the planes of separation between the commutatorbars for the purpose of preventing an instant of absolute rupture of the commutating-current when the trailing side of the live brush leaves the connnutator-bar, thereby avoiding a spark or burning of the bar. Another alternative means for accomplishing this end is shown in Figs. 4 and 5, in which the dead brushes are arranged in two series 7 h, the brushes being also staggered relatively to each other, so that a dead brush in one series partly overlaps a dead brush of the other series, while the live brush is dividedinto two parts, one being thicker than the other, so that one of the dead brushes may partly overlap it. Each series of dead brushes is not only insulated from the other series, but the individual dead brushes are insulated from each other, as in the main form of the invention.

It is obvious that for reversing motors dead brushes will be located at both sides of the live brush.

lVithout limiting myself to the details shown, what I claim, and desire to secure by Letters Patent of the United, States, is

The method of commutation, consisting in increasing step by step the number of brushcommutator contacts between the line and the advancing terminal of a coil which is being commutated, as the coil approaches the posi tion of a receding coil, thus finally forming a path of current, with maximum resistance, which leaves and returns a multiplicity of times into the commutator-surface, substantially as described.

Signed by me at Ampere, New Jersey, this 3d day of May, 1902.

GANO S. DUNN.

\Vitnesses:

F. V. HENsI-IAW, MARY E. SAMMIs. 

